The invention relates to a linear drive having a stator and a rotor that can move therein back and forth along an axis, with the stator having a magnetizable magnetic-field guide core that has legs extending each with one foot substantially toward the rotor; to a linear compressor, a refrigerating device, in particular a refrigerator and/or freezer or an air-conditioning system, as well as to a method for refrigerating an item and/or compressing a fluid.
Linear drives having permanently energized rotors are selectively moved by appropriately driving the drive coils on the stator by applying a suitable current to them. The coil fields are guided through a magnetizable magnetic-field guide core having poles and are focused onto a region around the rotor.
DE 699 015 86 T2 discloses an electromagnetic linear actuator comprising a stator construction having two poles excited by at least one electric winding. A moving component has a magnet yoke and a magnetizable part, with the moving component having one or two magnets that are permanently magnetized in a direction perpendicular to the plane of the air gap and set into a recess in the moving magnet yoke. A stator part is therein formed from a piece of ferromagnetic material having legs that have feet embodied as polar protuberances.
The problem posed by known solutions is that the rotor is difficult to drive precisely and in particular that its back-and-forth motion cannot always be controlled as precisely as desired. The imprecision of said back-and-forth motion is disadvantageous particularly in association with linear compressors because the compressor piston needs then to be positioned as accurately as possible to minimize a dead volume in the compressor-piston housing: The compressor piston will when moving in an uncontrolled manner strike the piston housing or a valve plate if a safety clearance between the piston housing and compressor piston has been underdimensioned, as a result of which the valve plate, the compressor piston, and the piston rod between the rotor and compressor piston may be damaged. Albeit said problem can be obviated by suitably increasing the safety clearance between the piston housing and compressor piston, the dead volume in the piston housing ought nonetheless to be as small as possible lest the linear compressor's operating efficiency be too adversely affected.
It is known how through a regulating means the imprecision in the back-and-forth motion can be corrected by placing positioning sensors on the rotor and braking it in a timely fashion by driving the drive coils appropriately. Although such braking operations can effectively prevent striking by the compressor piston, they are nonetheless disadvantageous given the need for as great as possible operating efficiency since the braking energy will be lost to the system.